Abstract

The present paper reports result of an experimental program conducted to study the behavior of geopolymer concrete at elevated temperature on the basis of physical appearance, weight loss and residual compressive strength test. The geopolymer concrete cubes of 150×150×150 mm were cast. Three cubes were tested for compressive strength at the age of 7 days and 28 days by universal testing machine. Then the specimen were subjected to the elevated temperature 200oc, 400oc, 600oc, 800oc and 1000oc in an electric air heated muffle and after cooling were tested for the compressive strength. Six cubes were immersed in each solution of sodium sulphate, sulfuric acid, and sodium chloride for 30 days and 60 days. The test reveal the properties of geopolymer concrete and its applicability at elevated temperature and against aggressive environment such as acid attack, sulphate attack and chloride attack.

Keywords

INTRODUCTION

Concrete usage around the world is second only to water. Ordinary Portland cement (OPC) is conventionally
used as the primary binder to produce concrete. The environmental issues associated with the production of OPC are
well known. The amount of the carbon dioxide released during the manufacture of OPC due to the calcination of
limestone and combustion of fossil fuel is in the order of one ton for every ton of OPC produced. In addition, the extent
of energy required to produce OPC is only next to steel and aluminum.

On the other hand, the abundant availability of fly ash worldwide creates opportunity to utilize this by-product
of burning coal, as a substitute for OPC to manufacture concrete. When used as a partial replacement of OPC, in the
presence of water and in ambient temperature, fly ash reacts with the calcium hydroxide during the hydration process
of OPC to form the calcium silicate hydrate (C-S-H) gel.

In 1978, Davidovits (1999) proposed that binders could be produced by a polymeric reaction of alkaline
liquids with the silicon and the aluminium in source materials of geological origin or by-product materials such as fly
ash and rice husk ash. He termed these binders as geopolymers. Palomo et al (1999) suggested that pozzolans such as
blast furnace slag might be activated using alkaline liquids to form a binder and hence totally replace the use of OPC in
concrete.

In this work, low-calcium fly ash-based geopolymer is used as the binder, instead of Portland or other
hydraulic cement paste, to produce concrete. The fly ash-based geopolymer paste binds the loose coarse aggregates,
fine aggregates and other un-reacted materials together to form the geopolymer concrete, with or without the presence
of admixtures. The manufacture of geopolymer concrete is carried out using the usual concrete technology methods.

LITERATURE REVIEW

Palomo et al (1999) studied the geopolymerisation of low-calcium ASTM Class F fly ash (molar Si/Al=1.81) using
four different solutions with the solution-to-fly ash ratio by mass of 0.25 to 0.30. The molar SiO2/K2O or SiO2/Na2O of the solutions was in the range of 0.63 to 1.23. The specimens were 10x10x60 mm in size. The best compressive
strength obtained was more than 60 MPa for mixtures that used a combination of sodium hydroxide and sodium silicate
solution, after curing the specimens for 24 hours at 65oC.

Xu and van Deventer (2000) reported that the proportion of alkaline solution to alumino-silicate powder by mass
should be approximately 0.33 to allow the geopolymeric reactions to occur. Alkaline solutions formed a thick gel
instantaneously upon mixing with the alumino-silicate powder. The specimen size in their study was 20x20x20 mm,
and the maximum compressive strength achieved was 19 MPa after 72 hours of curing at 35oC with stilbite source
material.

van Jaarsveld et al (1998) reported the use of the mass ratio of the solution to the powder of about 0.39. In their work,
57% fly ash was mixed with 15% kaolin or calcined kaolin. The alkaline liquid comprised 3.5% sodium silicate, 20%
water and 4% sodium or potassium hydroxide. In this case, they used specimen size of 50x50x50 mm. The maximum
compressive strength obtained was 75 MPa when fly ash and builders waste were used as the source material.

Davidovits (1982) and Barbosa et al (2000) prepared seven mixture compositions of geopolymer paste for the following
range of molar oxide ratios: 0.2<Na2O/SiO2<0.48; 3.3<SiO2/Al2O3<4.5 and 10<H2O/Na2O<25. From the tests performed on
the paste specimens, they found that the optimum composition occurred when the ratio of Na2O/SiO2 was 0.25, the ratio of
H2O/Na2O was 10.0, and the ratio of SiO2/Al2O3 was 3.3. Mixtures with high water content, i.e. H2O/Na2O = 25, developed
very low compressive strengths, and thus underlying the importance of water content in the mixture. There was no
information regarding the size of the specimens, while the moulds used were of a thin polyethylene film.

Teixeira-Pinto et al (2002) found that the fresh geopolymer mortar became very stiff and dry while mixing, and
exhibited high viscosity and cohesive nature. They suggested that the forced mixer type should be used in mixing the
geopolymer materials, instead of the gravity type mixer. An increase in the mixing time increased the temperature of
the fresh geopolymers, and hence reduced the workability. To improve the workability, they suggested the use of
admixtures to reduce the viscosity and cohesion.

Teixeira-Pinto et al (2002) concluded that Vicat needle apparatus is not appropriate to measure the setting time of
fresh geopolymer concrete, Chen and Chiu (2003) reported the only information available to date on the quantitative
measure of the setting time of geopolymer material using the Vicat needle. For the fresh geopolymer paste based on
metakaolin and ground blast furnace slag, they measured the setting time of the geopolymer material both at room and
elevated temperature. In the elevated temperature, the measurement was done in the oven. They found that the initial
setting time was very short for geopolymers cured at 60oC, in the range of 15 to 45 minutes.

Barbosa et al (1999) measured the viscosity of fresh metakaolin-based geopolymer paste, and reported that the
viscosity of the geopolymer paste increased with time. Most of the manufacturing process of making geopolymer paste
involved dry mixing of the source materials, followed by adding the alkaline solution and then further mixing for
another specified period of time (van Jaarsveld et al. 1998; Swanepoel and Strydom 2002; Teixeira-Pinto et al. 2002).

The chemical composition of the geopolymer material is similar to natural zeolitic materials, but the
microstructure is amorphous. Geopolymer material with sodium hydroxide and cured at elevated temperature will
attributed more stable cross- linked alumino silicate polymer structure. The properties and uses of geopolymers are
being explored in many scientific and industrial disciplines.

EXPERIMENTAL PROGRAM

A. Materials

The material used in present investigation were locally available in Sindri, Dist- Dhanbad(Jharkhand) and physical
properties were found through various laboratory tests conducted in Concrete and Road material lab, B.I.T Sindri.

III.A.1 Fine aggregate

Ordinary sand available in Sindri, Dhanbad (Damodar river sand) having the following characteristics has
been used.

Specific gravity : 2.67

Fineness modulus : 2.42

Unit weight : 1.674 gm/cc

Water absorption : 0.44%

Bulking : 26%

Sand after sieve analysis confirm to zone – II as per IS 383-1970.

III.A.2 Coarse aggregate

Locally available black crushed stone (Pakur stone) in Sindri with maximum nominal size of 20 mm and 10 mm
have been used as coarse aggregate. The physical properties for the coarse aggregate as found through laboratory test
according to IS 2386-1963 is resulted as:

Aggregate crushing value = 24%

Aggregate impact value = 29%

Specific gravity = 2.64

Water absorption = 0.94%

Unit weight = 1.60gm/cc

Fineness Modulus = 6.15

III.A.3 Fly ash

Low calcium fly ash samples taken from Bokaro Thermal Power Station, Bokaro (Jharkhand) were used in
this study. This fly ash was of average quality formed with the combustion of lignite and bituminous coal. The colour
of the fly ash was light grey. The sample satisfied the requirements of IS 3812(Part I).

The chemical characteristics are presented in table 2.4. The chemical property of the fly ash has been
presumed based on the data made available from Bokaro Thermal Power Station, Bokaro (Jharkhand).

III.A.4 Chemical Solution

A combination of sodium silicate solution and sodium hydroxide solution was chosen as the alkaline liquid.
Sodium-based solutions were chosen because they were cheaper than Potassium-based solutions. Both were
commercially available in market.

Sodium hydroxide

The sodium hydroxide (NaOH) solution was prepared by dissolving the pellets in water. The mass of NaOH
solids in a solution varied depending on the concentration of the solution expressed in terms of molar, M. For instance,
NaOH solution with a concentration of 8M consisted of 8x40 = 320 grams of NaOH solids (in flake or pellet form) per
litre of the solution, where 40 is the molecular weight of NaOH. The mass of NaOH solids for 97% purity was
measured as 260 grams per kg of NaOH solution of 8M concentration. Similarly, the mass of NaOH solids per kg of
the solution for other concentrations were measured as 10M: 314 grams, 12M: 361 grams, 14M: 404 grams, and 16M:
444 grams. Note that the mass of NaOH solids was only a fraction of the mass of the NaOH solution, and water is the
major component. Further through literature review it was strongly recommended that the sodium hydroxide solution
must be prepared 24 hours prior to use as it terminates to semi solid liquid state after 36 hours. The same
recommendation was adopted in the present work.

Sodium silicate

Sodium Silicate was available in semisolid form. The sodium silicate solution (Na2O= 14.7%, SiO2=29.4%,
and water=55.9% by mass) was purchased from a local supplier in bulk.

III.A.5 Water

Distilled water was used throughout the test procedure to achieve exact molarity.

B. Mix design of Geopolymer Concrete

As in the case of Portland cement concrete, the coarse aggregates and fine aggregates occupy about 75%-80% mass of
Geopolymer concrete. Assuming the aggregates to be in surface saturated dry condition and the unit weight of concrete
is 2400 Kg/m3. Combined aggregates are assumed to consist of 70% coarse aggregate and 30% fine aggregate. The mix proportion for 14 M geopolymer concrete is calculated and the value of different ingradients for one cubic meter
concrete by mass is given as:

C. Mixing and casting procedure

The specimen were prepared according to IS 516-1959. Mixing of all the material were done manually in the
laboratory at room temperature.The coarse aggregates, fine aggregates and fly-ash were weighed and placed on the
mixing floor, moistened in advance and mixed homogeneously. After mixing these ingradient, weight the water and
placed on the dry mix. simultaneously place the geopolymer binder which is homogeneous mixture of sodium
hydroxide and sodium silicate prepared 24 hrs prior tan mixing. The mixing of total mass was continued until the
binding paste covered all the aggregates and mixture become homogeneous and uniform in colour. Fresh concrete was
cast in steel mould and each cube specimen was cast in three layers by compacting manually(as shown in fig 2.2) as
well as by using vibration table as shown in fig.(2.3). Each layer received 35 strokes of compaction by standard
compaction rod for concrete, followed by further compaction on the vibration table. The cube specimens of size
150×150×150 mm size were used for compressive strength determination after demoulding at one day, the specification
were kept in hot air oven at 80oc until 24 hrs age and then cured in air with a temperature of 20oc and 50% relative
humidity. The mould were removed and kept at room temperature until the time of the experiment.

RESULTS AND DISCUSSION

The experimental results are presented and discussed. Each of the compressive strength test data plotted in Figures
or given Tables corresponds to the mean value of the compressive strengths. The effects of elevated temperature and
aggressive chemical environment on the compressive strength and weight loss of geopolymer concrete composite are
discussed.

A. Compressive strength at elevated temperature

Compressive strength of concrete is directly determined with the help of universal testing machine in concrete and
road material laboratory, BIT Sindri. The thermal stability of geopolymer concrete material prepared with sodium
containing activators rather low and significant changes in the microstructure occurred. At 1000oC,the strength of the concrete was reduced due to increase in the average pore size where amorphous structure were replaced by the
crystalline Na-feldspars. Fly ash based geopolymer prepared using class F fly as wit sodium silicate shows high
shrinkage as well as lare canes in compressive strength with increasing fire temperature in the range 800-1000oC.

The compressive strength for different conditions is given in table 4.1

Compressive strength against aggressive environment

Three numbers of specimens in each acid, sulfate and chloride solution were tested for compressive strength
after curing time and unit weight of specimens was noted before the test. Sulphuric acid attacks to lead to deposition of
a white layer of a gypsum crystal on the acid exposed surface of the specimen. The average test results are presented in
table 4.2 and corresponding diagram is plotted as shown in fig 4.2 for geopolymer concrete.

B. Loss of weight due to elevated temperature

The weight loss from geopolymer concrete composite increase with the increase in the maximum exposed
temperatures due to accelerated drying. Up to the temperature of 100oC, concrete specimens lost 0.3 % of either initial
weight due to the evaporation of free water. When the temperature was increased to 200oC, the weight loss was
0.972 %, and when the temperature was increased to 400oC, the weight loss was 2.855%. The increased weight loss is
probably due to the loss of water from the geopolymer concrete. The thermal expansion of geopolymer concrete
composite at elevated temperature is strongly affected by the expansion of aggregate because aggregates generally
occupy75-80% of volume of concrete. The expansion of aggregates usually predominates over the contraction of
geopolymer which produces a net result of expansion in the composite. At 600oC the concrete lost 3.94% of its
weight. At 800oC the weight loss was 5.81 % while at 1000oC the weight loss arrived to 8.5 %. The relationship
between the weight loss and maximum temperature is non- linear.

C. Loss of weight due to aggressive environment

Geopolymer concrete cubes were immersed in 10% concentration of sulphuric acid, 10% sodium sulfate solution
and 10% sodium chloride for test period of 30 days and 60 days. All the exposed specimen recorded weight loss
and it was observed that the weight loss in case of acid attack was less as compared to sulfate and chloride attack.
The results of change in weight is presented in Table 4.4

CONCLUSIONS

The experimental investigation confirms that:

• Loss of weight increased with increase in elevated temperature, weight loss was about 8.5% at 10000C.

• Loss of weight due to acid, sulfate and chloride effect is 0.6135 %, 2.5 % and 1.23 % respectively after
exposure of 30 days.

• Loss of weight due to acid, sulfate and chloride effect is 1.84 %, 4.37 % and 3.681 % respectively after
exposure of 60 days

• Sulphuric acid attacks lead lesser loss of weight and also deposition of a white layer of a gypsum crystal on
the acid exposed surface of the specimen.

• Geopolymer concrete showed reduction in strengths in compression when exposed to different aggressive
chemical environment. This reduction was about 6.92%, 2.145% and 2.645% after 30 days exposure and
10.581%, 4.075 % and 7.245 % after 60 days exposure to acid, sulfate and chloride solution respectively.

ACKNOWLEDEMENTS

The authors are grateful to Er. Shyam Deo Sharma & Er. Kumar Mani Bhushan (“BALAJI FLY ASH
BRICKS PVT. LTD, JEHANABAD”) for introducing them to the fascinating topic and for their advice,
encouragement and financial support.